This invention relates to a method of making improved ignition distributor rotor electrodes for internal combustion engines. The electrodes have dielectric material containing particles dispersed in a conductive metal for the purpose of substantially suppressing the generation of radio frequency signals. More particularly, the invention relates to making distributor rotor electrodes wherein the dielectric material retained in the RFI suppressing particles is present in the form of sponge-like microscopic bodies with copper metal in the pores. When such particles are sintered with additional metal powder, the copper from the particles fuses therewith so that the sponge-like dielectric bodies are strongly mechanically interlocked with a substantially continuous metal matrix.
Radio frequency interference (RFI) in vehicles powered by spark ignition internal combustion engines is caused, for the most part, by high voltage spark discharges across the rotor gap. The RFI source is a large, fast rise time impulsive current generated at the onset of the rotor gap breakdown. The higher the voltage required to break down the gap, the higher the intensity of the radio frequency interference noise. It is known that the breakdown voltage across a given rotor gap can be lowered by providing a source of initiatory electrons, e.g., a dielectric material, at the surface of the rotor electrode at the gap. It is theorized that these electrons are accelerated away from the electrode tip by the application of an electric field to collide with air molecules in the gap and ionize them. The presence of these ions increases the probability that the rotor gap will break down at a lowered voltage where RFI is substantially suppressed.
Certain prior art RFI suppressing rotor electrodes comprised sintered mixtures of dielectric silica or glass particles, in the form of generally point symmetrical round powder or line symmetrical short fibers, and metal powder. Although such electrodes would suppress RFI noise generation, sparking at the rotor gap could dislodge the regularly shaped dielectric particles and cause excessive electrode wear, noise in the FM frequency band, and shortened service life.
It is an object of this invention to provide a novel two phase (dielectric phase-conductive metal phase) material which, when incorporated in an ignition distributor rotor electrode, provides durability and substantially suppresses radio frequency interference. It is a more particular object to provide an RFI suppressing rotor material wherein dielectric bodies of silica or silica and alumina are present in the material in the form of sponge-like or lacy dielectric elements with copper metal filling in the interstices. It is another object to provide a method of making and of incorporating such two phase material into a metal rotor electrode by substantially fusing the copper from the RFI suppressing material with additional metal powder to form a substantially continuous metal matrix in which the sponge-like dielectric elements are securely retained. Our rotor electrodes are highly resistant to spark erosion and wear due to improved retention of the dielectric element in the matrix metal.